From EMGs FAQ:
"Yes. If you've got room for multiple batteries in your guitar, you can use two batteries wired in series to power your onboard circuitry at 18 volts. The output level will not appreciably increase, but you'll have increased headroom and crisper transients. This is especially useful for percussive/slap bass styles where you can generate enormous instantaneous power levels across the entire frequency spectrum.
You can also wire two batteries in parallel to provide a regular 9 volt supply but with much longer lifespan between battery changes.
Although most of our products are rated for 27 volts, we recommend a maximum of 18 volts. The additional benefits of 27 vs. 18 volts are negligible."
Basically more voltage, more sensitive, more headroom, not a lot more gain.